1. Field of the Invention
The present invention relates to a bump forming apparatus and a bump forming method for forming a bump on a land of a substrate or a pad of a chip.
2. Description of the Background Art
FIG. 15 is a sectional view typically showing the structure of a conventional bump forming apparatus (refer to Japanese Patent Laying-Open Gazette No. 3-60036 (1991)). An ink jet printer 121 has a head 123, which is provided therein with a paste chamber 124 and an air chamber 125. The paste chamber 124 is provided with a plurality of inner nozzles 126, while the air chamber 125 is provided with outer nozzles 127 on portions opposite to the inner nozzles 125 respectively. A plurality of control electrodes 128 are provided on a lower surface of the paste chamber 124 around the inner nozzles 126. A plurality of common electrodes 129 are provided on a lower surface of the air chamber 125 around the outer nozzles 127.
A method of forming bumps with the ink jet printer 121 shown in FIG. 15 is now described. First, a semiconductor element 130 is arranged oppositely to the head 123. More specifically, the semiconductor element 130, which is provided with a plurality of electrode pads 131, is so located that the plurality of electrode pads 131 are opposed to the plurality of outer nozzles 127 respectively.
Then, compressed air is externally supplied into the air chamber 125 as shown by arrow X. The pressure of the compressed air is set to about 0.5 kg/cm.sup.2 to several kg/cm.sup.2, for example. At this time, the paste chamber 124 holds conductor paste 122 therein. The conductor paste 122 consists of metal powder, a binder and an adhesive. The metal powder, which is chosen in consideration of the materials for bumps 132 to be formed and the electrode pads 131 or the like, is prepared from powder of solder, tin or lead. The binder, which is employed as a solvent, is prepared from volatile methyl alcohol, isopropyl alcohol or methyl ethyl ketone. The adhesive is prepared that having a flux action. The compressed air supplied into the air chamber 125 forms air flows Y to flow out through the outer nozzles 127.
Then, voltage of about minus several 100 V is applied between the control electrodes 128 and the common electrodes 129. Due to electrostatic force resulting from this voltage application, the conductor paste 122 is extracted in a constant amount from the paste chamber 124 successively through the inner nozzles 126, the air chamber 125 and the outer nozzles 127. At this time, the air flows Y guide the conductor paste 122 to the outer nozzles 127 and quickly push out the conductor paste 122.
The conductor paste 122 discharged from the outer nozzles 127 splashes in the form of thin threads of about 10 .mu.m to several 10 .mu.m in diameter, for example, to adhere onto the electrode pads 131 of the semiconductor element 130. FIG. 15 sequentially illustrates conductive paste 122a starting to be discharged, discharged conductor paste 122b and conductor paste 122c starting to adhere onto one of the electrode pads 131. Finally a bump 132 is formed on each electrode pad 131.
However, the conventional bump forming apparatus, employing the ink jet printer 121 while simply using the conductor paste 122 in place of ink, has the following problems:
First, the inner and outer nozzles 126 and 127, which are readily clogged with the conductor paste 122 containing the metal powder of solder or the like and the adhesive, tend to defectively discharge the conductor paste 122.
Further, the conductor paste 122 has high viscosity due to the adhesive contained therein, while the air chamber 125 applies pressure of about 0.5 kg/cm.sup.2 to everal kg/cm.sup.2 to the inner nozzles 126. Therefore, it takes a long time after the conductor paste 122 is temporarily discharged from the inner nozzles 126 until new conductor paste 122 fills up the inner nozzles 126. Thus, it is difficult to discharge the conductor paste 122 at a high speed, while the amount of discharge is dispersed if the conductor paste 122 is discharged in a state incompletely filling up the inner nozzles 126.
Further, since pressure is applied to the inner nozzles 126 from the air chamber 125, air enters the paste chamber 124 from the air chamber 125 through the inner nozzles 126 when the conductor paste 122 is discharged from the inner nozzles 126 to form bubbles, which cause unstable discharge of the conductor paste 122.